1. Field of the Invention
The present invention relates to a method for producing nitrogen trifluoride, and more particularly to a method for producing nitrogen trifluoride using jet-loop reactors, wherein fused ammonium fluoride salt is rapidly ejected through a nozzle so that the fluorine gas is sucked and well mixed in the fused ammonium fluoride salt ejected as a fast stream, thus producing nitrogen trifluoride in a high yield.
2. Description of the Prior Art
Nitrogen trifluoride (hereinafter called NF3) is a stable gas, that has little reactivity at room temperature. It is an expensive gas, which has been gradually and widespreadingly used in etching of silicon or silicon oxide layers and in dry cleaning of chemical vapor deposition (CVD) chambers in the manufacturing processes of the semiconductor or the liquid crystal display (LCD). It has been also used as a source of chemical laser with stable high-energy, a rocket fuel, a gas for surface-modification of organic/inorganic materials such as polymers, a source of fluorine in producing fluoroolefins, and a dehydration gas in the optical fiber manufacturing process.
It has been known that NF3 can be produced by various methods: first, a reaction through the contact of fluorine gas with a fused ammonium fluoride salt; second, collection at an anode through an electrolysis of a fused ammonium fluoride salt, a mixed fused salt of ammonium fluoride and potassium fluoride, or a mixed fused salt of ammonium fluoride and urea; third, a reaction of inorganic ammonium salt with metallic fluoride such as NiF2; and fourth, plasma reaction between fluorine and nitrogen at high temperature.
U.S. Pat. No. 4,091,081 (1978) discloses a method for producing NF3 by directly contacting fluorine gas with a fused ammonium fluoride salt, wherein the fused ammonium fluoride salt is intensely stirred by a flat-blade turbine and fluorine gas is supplied thereto through a sparger with small discharge holes, thereby the gas and the fused salt contact with each other. Though the document discloses that the yield of NF3 reaches 63% which is larger than 30˜50% that the former conventional documents have announced, it has a drawback in that the yield is gradually reduced with the passage of time.
U.S. Pat. No. 5,637,285 (1997) discloses a method for producing NF3 by supplying fluorine gas while intensely stirring a fused ammonium fluoride salt or a metallic fluoride-containing fused ammonium fluoride salt with a flat-blade turbine, thereby maintaining the selectivity of NF3 at 60% or higher and the yield at 65% or higher. However, this method requires a strong stirring force per unit volume of at least 4.5 kW/m3 for converting F2 up to 63%, so that the NF3 yield is not satisfied even while introducing an intense stirring force many times larger than that used in ordinary chemical reactors. It is understood that this is because of the low contact efficiency of the fused ammonium fluoride salt and F2.
U.S. Pat. No. 4,543,242 (1985) discloses that, in a laboratory study, F2 gas is contacted with a granular solid metallic ammonium fluoride complex such as (NH4)3AlF6 or (NH4)2NaAlF6, etc. at a temperature of 80° C. or higher, thus producing a NF3 yield of 65˜78%.
Japanese Patent No. 44,212 (2000) improves the above-mentioned method, thus increasing the NF3 yield up to 87%. However, the solid reactants should be periodically replaced with new ones because they are difficult to be regenerated. In addition, there is no proper method for safely discarding the solid waste matters. Therefore, this method is not yet adopted industrially.
U.S. Pat. No. 3,304,248 (1967) discloses a method for producing NF3 by reacting plasmas of nitrogen and fluorine at a temperature of 8000° C. or higher and then rapidly cooling the gases.
U.S. Pat. No. 3,235,474 (1966) and U.S. Pat. No. 3,356,454 (1967) disclose a method for collecting NF3 at an anode made of amorphous carbon or Ni through the electrolysis of a fused ammonium fluoride salt. This method, however, has a drawback that since an explosion occurs if NF3 generated from an anode is mixed with H2 generated from a cathode at a certain ratio, the concentrations of NF3 and H2 should be controlled in a certain range not to cause an explosion by adding a large amount of an inert gas such as He or nitrogen thereto.
In order to prevent an explosion during an electrolysis, U.S. Pat. No. 4,804,447 (1989) discloses a method for producing a H2-excluded NF3 gas by electrolyzing a fused mixed solution of ammonium fluoride salt and N2H4.2HF, thereby obtaining about 55% of NF3 yield.
In order to prevent an explosion during the electrolysis, other U.S. Pat. No. 5,084,156 (1992) and U.S. Pat. No. 5,085,752 (1992) disclose an electrolyzer using a skirt made of or coated with fluorine-based resin, which still has drawbacks that the Ni anode is electrically eroded during the electrolysis to gradually produce metal sludge.
It has been known that, among the above methods for producing NF3, a direct fluorination, in which a fused ammonium fluoride salt is reacted with fluorine to produce NF3, and an electrochemical fluorination, in which a fused ammonium fluoride salt is electrolyzed to produce NF3 on the anode side, are commercially useful. The direct fluorination the present invention pertains to will be hereinafter described in detail.
As described above, the direct fluorination is a method, wherein fluorine is introduced into a fused ammonium fluoride salt so as to directly contact the ammonium fluoride salt. When introduced, fluorine reacts with the fused ammonium fluoride salt, thus producing NF3 and hydrofluoric acid anhydride (hereinafter referred to HF) as shown in the following reaction equation 1.NH4F+3F2→NF3+4HF  {reaction equation 1}
However, since the fused ammonium fluoride salt is reacted in the state in which HF has been bound thereto and the HF produced during the reaction becomes additionally bound to the ammonium fluoride salt (the binding method is not definitely defined) without being vented as HF vapor, the overall reaction equation of the direct fluorination can be expressed as the following reaction equation 2.mNH4F.xHF+3F2→NF3+(m−1)NH4F.(mx+4)/(m−1)HF  [reaction equation 2]
Herein, x is a number of 0˜5, m is an integer larger than 1. When m is 1 and x is 0, the reaction occurs as the reaction equation 1, so that, in general, 4 moles of HF per one mole of NF3 are generated and are bound to NH4F, whereby the number of HF bound to one mole of NH4F is expressed as (mx+4)/(m−1) which exceeds x.
Accordingly, along the procedure of the reaction, the number of HF to be bound to the ammonium fluoride salt is gradually increased, so that, in order to constantly maintain the number of HF bound to the ammonium fluoride salt, ammonia needs to be introduced continuously or intermittently so as to react with HF. Such a reproduction reaction of the ammonium fluoride salt as above is expressed as the following reaction equation 3.nNH4F.yHF+NH3→(n+1)NH4F.(ny−1)/(n+1)HF  [reaction equation 3]
Herein, y is a number higher than 0 and lower than 5, and n is a large integer of 1 or more. When the reaction is proceeded, the number of moles of NH4F is increased and thus the number of HF bound to one mole of NH4F, which is expressed as (ny−1)/(n+1), is lower than y.
Meanwhile, both the production reaction for NF3 through reacting fluorine in gaseous state with a fused ammonium fluoride salt (reaction equation 2) and the reproduction reaction for the ammonium fluoride salt through reacting ammonia with the fused ammonium fluoride salt bound with extra HF (reaction equation 3) have drawbacks that their reaction rates are so rapid and their heats of reaction are so high that it is difficult to control their reaction temperatures due to the rapid rise of temperature during the reactions.
Thus, in the direct fluorination, the problems to be solved for commercialization are concerned in the ways for supplying gaseous fluorine and ammonia to a fused ammonium fluoride salt while being smoothly and homogeneously distributed therein and for maintaining a constant amount of the fused salt in a reactor through removal of the extra amount of the ammonium fluoride salt produced.
For further reference, a net reaction equation representing both the production reaction for NF3 (reaction equation 2) and the reproduction reaction for a fused ammonium fluoride salt (reaction equation 3) can be expressed as the following reaction equation 4. Yield (%) of NF3 can be calculated by the following calculation equation 1 with reference to moles of F2 supplied.F2+4/3NH3→1/3NF3(object)+NH4F(extra ammonium fluoride salt produced)  [reaction equation 4][NF3yield, %]=100×[(moles of NF3produced)×3]/[moles of F2 supplied]  [calculation equation 1]